


Walk-Ins Accepted

by maximum_overboner



Category: Undertale (Video Game)
Genre: Banter, Frottage, It's porn folks we all know why we're here, Lighthearted, M/M, Masturbation, Porn, ectocock, gaster has a tentacle dick, handjobs, smutfic
Language: English
Status: Completed
Published: 2016-09-10
Updated: 2016-09-10
Packaged: 2018-08-14 05:21:41
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,479
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8000107
Author URL: https://archiveofourown.org/users/maximum_overboner/pseuds/maximum_overboner
Summary: <blockquote class="userstuff">
              <p>Sans and Gaster are staying in a hotel, attending and speaking at various Monster peace talks, when Gaster walks in on something very… Private.</p><p>And when presented with such an opportunity, it's a shame not to capitalize on it.</p>
            </blockquote>





	Walk-Ins Accepted

**Author's Note:**

> an anon requested gaster walking in on sans whacking it, so i hope you guys dig this too. love me some sanster.

 

“how long’re these meetings going to last, gaster?” **  
**

‘Not long, I would think. The bulk of it is out of the way.’

“phew, good.”

‘Another week, or so.’

“ugh.”

Gaster loomed, staring out of the window over the city, new and bustling and, though he would not admit it, intimidating. Huge, towering buildings, filled with lights and technology years ahead of what they had been confined to in the Underground, as a scavenger species. He basked in the shadow of the skyscraper opposite, it draping a cooling cloak over the building, cutting through the summer heat. It was early morning, and his movements had woken Sans.

‘I would like to live in a building like this. A tall one.’

Sans blinked dozily, before a thought set in. “same. i could hawk loogies from the top and nobody would know who’s doing it.”

‘I… Was going to make some sort of statement about looking down on a world that looked down on me, but... I mean, really? That was your first thought?’

“it’s as good a reason as any. papyrus could live it up on the roof, too. get one of those big mirrors, plonk himself on a towel and bleach his bones from the comfort of home.”

Sans tapped his fingers absentmindedly together, before coughing, and Gaster picked up on it.

‘I know you miss your brother,’ Gaster soothed, ‘but you’re needed here with me. I can’t take all of the credit, you are my assistant after all.’

“i said you could.”

Like a pillar, casting a faded shadow, Gaster stood, still and forever. ‘It’s not a case of saying that I can, it’s a case of participating.’

Sans sighed, giving up the point of contention before it could even really get going, and flopped backwards onto his bed. At least this was comfy. He splayed his arms, getting cosy, warm, ready to sleep.

“is it really necessary, though? do you want all the attention, the… dudes taking pictures?”

‘Pictures, I can handle. Being looked at as a sideshow, I cannot. But it turns out that building a huge, heat-based power plant out of nothing but scraps, manpower and necessity, while also being the most educated person to ever exist regarding magic, a subject thought to be nonexistent up here until last month, is somewhat of a big deal. We need to participate in talks, no matter how long, or how dry, they may be. For Asgore’s sake. I’m not sure if he’s in the state to be dealing with this given the probing questions he’s been asked about his children. It’s best that we do, and spare him the trouble. I owe him that much, and by default, you owe him that much.’

Sans went quiet, lying flat on the bed.

“long as you need me,” he said, his voice lacking the casual edge Gaster was accustomed to, a statement, rather than a response, “i’ll stick around.”

‘Good. It’s what you’re paid for, after all,' he added dryly.

Sans rolled onto his front, pyjama shirt riding up, and groaned in exasperation. “the meetings have been so awkward, dude.”

‘I’m aware.’

“it’s been so bad.”

‘Indeed.’

“i mean, we aren’t that funky looking, are we?”

Gaster gave him a withering, if sympathetic, look. ‘You are a skeleton, the thing they become when they die, and I’m… Unsightly.’

“aw, you're not unsightly. you’re… like, sittin’ on a good ‘what the fuck is that’, but you aren’t unsightly.”

‘Would you believe me if I said that’s the closest anyone has come to complimenting my appearance?’

Sans winced, looking Gaster up and down, who drifted over and sat on his own bed so he could be better seen. “ouch.”

A silence drifted between them, with no movements to cut through it. It was not unpleasant. It was a companionable sort.

‘I’ll go for a walk, I think. I’m fond of those.’

“in the city? won’t you get… like, accosted?”

‘Oh, what are they going to do, stab me? They wouldn’t get the knife back.’

“good point.”

‘You’re welcome to come along, if you wish. There’s nothing on the agenda for today, and I could use the company.’

“that’s kind of ya, but no thanks. i’ve got some high intensity snoozin’ waiting for me. gotta get myself prepped. unless you need me to come with and translate?”

‘I think I will be fine, I have nothing really planned, and I’m not going to go out of my way to speak to anyone. If you want to be left to it, then I understand. Have fun with your… High intensity snoozing.’

Sans yawned, his jaw popping and clicking, before he settled himself back into the warm confines of his blanket, blinking dozily.

“you need me, just phone me and i’ll--”

Gaster looked at him flatly, brows knit together. There was a pause, before he signed.

‘I will text you.’

“yeah, that will go better.”

‘What about you, do you plan to sleep the entire day?’

A glint passed across Sans’ eye, mirthful and wicked. “nah, i might head out later on. it’ll be good to break my dry spell.”

Gaster felt his heart go cold, out of his repressed attraction to Sans, and for the inevitable humiliation that his next question would bring. But his curiosity regarding Sans’ sex life won out, and he asked anyway, bracing himself.

‘Out of curiosity… What do you consider a ‘dry spell’?’

“it’s been, uh...god… hey, when was that halloween party, the office one? when the cleaner and that structural engineer got drunk and beat the shit out of each other over a muffin.”

‘Two years ago.’

“yeah, it was before that. about three years. random chick walked up to me outside a bar and put my hand on her boob and i was like ‘oh damn i can roll with this.”

Gaster blanched, but his face was so pale that it was barely noticeable.

‘Three years is a dry spell.’

“yeah.”

‘Three years.’

“uh huh. fuck, that’s not that long, is it?”

Sans’ gaze wandered up and down Gaster, before splitting into a gently-mocking smile, sensing something bubbling in the undercurrent of their conversation. “why?”

Gaster fidgeting, and that was when Sans knew he was unearthing something that perhaps Gaster was not comfortable with. Not that this stopped him, however, when an opportunity to prod would present itself, especially with the prideful doctor, then he would grasp onto it with both hands. The hesitation forced Gaster to answer, as if he were to just dissipate and leave, Sans would probably think it was far worse than it actually was.

‘... No reason.’

“i told you about mine, how bad can yours be?”

‘Take your number.’

“three years, ok, i’m with ya.”

‘Then times it by ten.’

“... thirty years? oh my god. a dry spell is a ‘time without sex’, this is… i mean, you’d be dead of dehydration.”

‘I kn--’

“like a million times over.”

‘Are you quite finished?” Gaster chided, face tinting in a manner Sans could only describe as hilarious. ‘Or is this going to devolve into a discussion about my sexual habits, because I would rather it didn’t.’

Sans was narrowing his eyes with triumphant mirth, digging into the veins of the topic, picking at Gaster’s usually unflappable demeanor.

“ahh, so you can quiz me on my screwing quota, but when i bring yours up it’s a step too far, right?”

Gaster leaned forward, preparing a rebuttal, looking at Sans’ smug, chubby face.

‘... Point made. Now if you’ll excuse me, I think I’ll leave before I humiliate myself any further.’

“fair. have fun scaring the shit out of humans.”

‘Will do.’

Gliding with purpose over the carpet towards the door, he let himself out, making sure not to look Sans in the eye as he did so, because dammit, he could feel him smirking at his back.

From that point on, it was a case of making his way to the lobby, and thus, out of the building. Taking the elevator seemed… Inadvisable. Every window he passed was a stark reminder of his own appearance, a unique brand of horror that seemed to tap into the primordial fear of every Human that gazed upon him, uncannily close in stature but viscous, and melted, as if diseased. He would compromise, he didn’t want to instigate conflicts, especially considering his status as nobility. Asgore would have quite a few words for him if he were to antagonize anyone at this stage, and so he would obey.

He took the stairs. And what a sight it was.

His stalk was not meant to descend stairs, and the process took him fifteen minutes. He made a mental note to just throw himself over the bannister next time, then reconstitute after the harmless impact, to hell with any horrified onlookers. He had places to be. Not at that exact moment, perhaps, but in the future, probably. He was very important after all. And important people threw themselves down the stairs sometimes, it was just common sense.

Finally, the cobalt sky lightened to a blue, with murky pinks streaked across its bulk like paint across canvas, and he reached the lobby. He glided past a baffled receptionist, giving her a curt nod, before his gaze drifted to a nearby calendar, hanging on the wall behind her. He braced his palms to his face with such force that he may as well have struck himself.

Thursday. They had nothing planned on Thursday.

Today was Wednesday, and if he recalled correctly, they had to attend a conference at eleven o’clock. And Sans wouldn’t respond to his messages, he knew, of course he did, Sans barely responded to his own name at times, and his circumstances forbade calling.

With a groan of frustration, Gaster boarded the elevator, and returned to his floor in a blistering forty seconds. He was in the mood for a conflict, and found none.

Silently, though quickly, his hand found the door.

Gaster swung it open with purpose.

Sans was fucking himself on the bed. His ass was in the air, pyjama bottoms slipped down entirely now that he was delighting in the privacy he had been missing out on in their shared room, wetly fucking his palm and moaning with every spasm. His hips rocked, thrashing forward to meet his stationary grip, the sounds growing louder as he lost himself in it, in the sensation. It had only been a few weeks since he had last cum, but he hadn’t been able to find privacy, they had both been so busy--

Gaster stood slack jawed, entirely still, unmoving, eyes fixated on Sans’ motions. The light hit Sans’ half open sockets, and he rocketed his gaze towards its source, Gaster standing in silhouette.

They stared.

And stared.

And stared, as the air became thicker. Sans still had his hand locked around his cock, stunned, his pelvis still hovering, while Gaster still had his hand braced, frozen, before haltingly removing it to sign.

‘Oh. Hello.’

“... uh… hey…”

‘... How… How are you?’

“... good?”

‘Good. I’m glad you’re… You’re good.’

“uh… yeah… same.”

Gaster was frozen in the doorway, unsure of how to proceed, face on fire, mirroring Sans’ blush.

“are you, uh… can you shut the door, dude?”

‘Hmm? I-- right, of course--’

“it’s just that people can see me from the hall, so--”

‘Say no more, that’s perfectly reasonable--’

Gaster stepped inside, before gently shutting the door with a ‘click’, and it was only when he met Sans’ gaze again that he had realized his error.

“... you-- i mean-- i thought you were headin’ back down--”

‘Oh God.’

“i was kinda implyin’ that you-- i thought--”

‘No-- Of course, I--’

“why’d you walk _in?_ ”

Gaster didn’t have an answer that let him keep the few thin threads of his dignity, and so declined to do so. He couldn’t tear his gaze from Sans’, wild eyed and aroused, deep in the throes of self pleasure, fucking himself.

Gaster felt a familiar heat.

No.

_No._

_If God was real, then this would not be happening._

To his own mortification, as well as Sans’, his erection unfurled, slowly writhing and searching for stimulus, thick at the base and tapered at the end. Sans had crammed his face into a pillow, accepting death.

“oh you’ve got to be shitting me.”

Gaster had been around a very long time. He had seen people be born, live, and die without making an impact on the world. He had seen famine ravage those around him, and not blinked. He had seen people suffer needlessly, and die needlessly. But he could say, without any hesitation, that this was, by far, the worst thing that had ever happened, to anyone.

Sans tore his gaze back, now hiding his pelvis with the pillow, though Gaster couldn’t help but notice the faint blue light that seeped through it.

“just… dude, get rid of it!”

‘I-I can’t. Not without-- I-- I can’t go outside like this. It’s indecent.’ Gaster could only bashfully hide his own erection, accidently grazing it and making his situation far worse.

Sans stole glances, his own hardness not dissipating, and he moved his weight from knee to knee, unable to deny the sexually charged atmosphere that weighed down on both of them. He couldn’t even blame Gaster, his reaction was a normal one to sexual stimulus, but he could sure as hell be mortified all the same.

‘Look! Look. We’re both reasonable adults. There’s nothing wrong with masturbation, it’s a perfectly healthy way of relieving stress.’

“oh my god.”

‘And, with that in mind, I don’t think there should be any shame surrounding a natural act--’

“dude.”

Another silence befell them both, neither of them any less aroused than before. Taking a deep breath, Gaster stormed over, and for a brief second Sans was worried he was going to be pushed.

“whoah--”

Gaster grabbed Sans’ burgeoning cock, holding it still while Sans processed what was unfolding, both of them bathed in natural light.

“that’s… that’s my dick.”

Gaster nodded. He spelled with his free hand. He may as well grasp the opportunity, he thought, it certainly couldn't make things more awkward. Gently, he pushed Sas to lie flat on the bed, and he complied totally.

‘S-T-O-P?’

“i-- i mean-- you’re already here, so… i-if you’re cool with it--”

Gaster violently jerked Sans’ cock, before pumping it furiously, expressing his own sexual frustration in his movements. Face set in concentration he lowered himself onto the bed, before slithering up to straddle Sans, stalk off to one side.

“whoah, you-- you’re pretty heavy--”

Gaster, with trembling palms, looking Sans in the pit of his deep, dark eyes, brought his undulating, tentacle-like cock to Sans’, and let out a hiss of pleasure as it furled and intertwined with his more rigid member. Sans threw his head back into the pillow, the novelty of the slick, wet sensation forcing squeaks of pleasure out of his fixed smile, his fingers gripping and twisting at the sheets tightly enough to hurt.

This was weird. A good sort, but still. And he could tick off ‘figure out what’s up with gaster, does he have a penis, or five, or something weird’ off of his bucket list. It was a productive day.

Sans brought his hand to work their intertwined cocks, and Gaster almost cooed. Such small hands. They were barely doing anything, his fingers seemed to struggle against the girth of his own cock, nevermind two.

Bringing his own hand down in turn, Gaster laid it over Sans’ in a moment that, despite the situation they were in, was precariously intimate, and pumped in tandem, slicking them both until they established a steady rhythm. In a show of boldness, Gaster brought his forehead to clack gently against Sans’ skull, to take in his vacant, lustful expression, and taste his breath, heavy and musky from the morning. Sans looked back and, after some consideration, brought his mouth to connect with Gaster’s in a desperate, sloppy kiss, tongues bumping and teeth clashing, though one that got the point across.

Gaster groaned quietly into Sans’ mouth, bed creaking softly and slowly as he instinctively pumped his hips to grind their cocks together, his erection working dutifully at Sans’, bringing them both closer and closer to the edge, God, how scandalous, how absurd. Sans was probably only going along with this out of his immediate lust, Gaster knew, and so he was willing to milk the moment for all it was worth. The breathing, the panting, Sans’ sweat-flushed bones creaking underneath him as his face scrunched and contorted in bliss. It almost as looked as if he was in pain.

“please,” he croaked, almost drowned out by the wet pump of Gaster’s prehensile, eager cock against his own phantom flesh, rapid and pulsing as they desperately ground into each other.

Gaster dipping his head lower, into the crook of Sans’ shoulder, to hear him better, to hear every near-imperceptible creak and groan.

“w--wanna c-cum--”

With a primal growl, Gaster sped his movements until they bordering on painful, rutting his hips and his writhing, pulsing cock against Sans’ as hard and as fast as he could, sweat dripping from his face as his arms ensconced his small frame, rutting with a violent force that made the bedsprings moan and squeal, bobbing Sans up and down, it was too much, too much, the novelty, he couldn’t keep up.

Sans came first. He had a head start. It was only fair.

His deep voice cracked, giving way to a moan that slipped and hiccupped, until it was a barely coherent stream of words as he pumped out his huge load; syllables that served to make a point rather than convey a meaning.

“c-- cummin’-- cum-- fuck-- f-fuck--”

Gaster reared his head back, eyes squeezed shut, increasing his force once more, so close, so, so close, agonizingly, standing on the precipice but not able to--

“g-- gaster--”

His eyes rocketed open as he froze, before his body twitched rhythmically as the orgasm pumped through him, exuding from his core, making his entire frame spasm as he shot out ropey streams of cum in irregular patterns. His mouth was open, and tears were beading his eyes from the incoming stimulation.

“S-- San--”

Sans pumped his closed fist, riding out the last vestiges of his orgasm, before he finally fell limp, exhausted, and admittedly, confused. Gaster followed soon after, his erection vanishing back into his body after it was spent.

They lay there, tired, Gaster hunched into Sans, while Sans was too drained to willingly break the embrace. The warm weight was soothing on him, and he was more than ready for that nap now. Despite his sleepiness, however, it was best to ask his questions now, lest it become awkward.

Well… Even more awkward. Tapping Gaster on the shoulder and receiving a very weary, very flushed, look, he spoke.

“so do… you always walk up to your staff and start dishin’ out handjobs like rice krispy squares, or… i mean, i’m not complainin’ but you went from zero to a hundred,’ he wheezed.

Best to be… Somewhat honest. Gaster saw an opportunity, and capitalized on it. He certainly wasn’t going to retire to the bathroom to masturbate when Sans was right here, after all, and so it might as well have been a joint affair.

‘I… Was thinking to that woman in the bar, the one you were talking about, and--’

Sans looked back, wide eyed, breathing heavily, mouth hanging open just enough to let Gaster see the pointed tips of his teeth.

‘I… Have no boob. And so I had to make do.’

“... you ‘have no boob’?”

‘ _You know damn well I’m out of practice._ ’

Sans let his gaze drift to the ceiling, vision drifting in and out as his hands slowly trickled from Gasters hip, to his shoulders, in something that almost resembled a lovers hold.

“you could have asked instead of just… hoppin’ in. again, not that i mind.”

‘I would like to think that I carry myself with greater grace than random, horny people at a bar, but… Apparently not. Though, that being said, you seemed quite pleased. Both in this case, and the previous, I should note.’

“how dare you question the sanctity of bar titties like this. they were a gift, gaster. what, would i have said no. to bar titties? out of your mind”

‘Yes, yes.’

“bar titties, man.”

Gaster let loose a low, deep chuckle, settling into a post-sex afterglow he had almost forgotten about, that lifted his mood and lightened his spirits. This was warm, and comfortable, if… Somewhat difficult to quantify.

“least i can nap now.”

‘You can’t,” Gaster signed, chest heaving, ‘It’s why I came up here, I erroneously planned for the wrong day. We have a packed schedule.’

“oh _come on_.”


End file.
